- Technical Summary
This draft describes how an Ethernet Pseudowire (PW) is used to
carry Ethernet/802.3 Protocol Data Units over an MPLS network.
This enables service providers to offer "emulated" ethernet
services over existing MPLS networks. This document specifies
the encapsulation of Ethernet/802.3 PDUs within a pseudo wire.
It also specifies the procedures for using a PW to provide
a "point-to-point ethernet" service.
- Working Group Summary
This work has been thoroughly analysed by the working group
and there is consensus for the design. A complementatry
specification exists in L2TPext WG which describes how to
carry an Ethernet PW over L2TPv3 over IP.
- Protocol Quality
This specification is well known in the industry and
implementations exist.
Note to RFC Editor
At the end of Section 2, please add the following sentence:
"Additional terminology relevant to pseudowires and Layer 2 Virtual Private
Networking (L2VPN) in general may be found in [RFC4026]."
Please replace the first three paragraphs of Section 4.6 with the following
text:
The Control Word defined in this section is based on the
Generic PW MPLS Control Word as defined in [PWE3-CW]. It
provides the ability to sequence individual frames on the
PW, avoidance of equal-cost multiple-path load-balancing
(ECMP) [RFC2992], and OAM mechanisms including VCCV [VCCV].
[PWE3-CW] states, "If a PW is sensitive to packet misordering
and is being carried over an MPLS PSN that uses the contents of the
MPLS payload to select the ECMP path, it MUST employ a mechanism which
prevents packet misordering." This is necessary due to the fact that
ECMP implementations may examine the first nibble after the MPLS label
stack to determine whether the labelled packet is IP or not. Thus,
if the source MAC address of an ethernet frame carried over
the PW without a control word present begins with 0x4 or 0x6, it
could be mistaken for an IPv4 or IPv6 packet. This could, depending
on the configuration and topology of the MPLS network, lead to a
situation where all packets for a given PW do not follow the
same path. This may increase out-of-order frames on a given
PW, or cause OAM packets to follow a different path than actual
traffic (see section 4.4.3 on Frame Ordering).
The features that the control word provides may not be
needed for a given ethernet PW. For example, ECMP may not
be present or active on a given MPLS network, strict
frame sequencing may not be required, etc. If this is the
case, the control word provides little value and is therefore
optional. Early ethernet PW implementations have been deployed
that do not include a control word or the ability to process one if
present. To aid in backwards compatibility, future
implementations MUST be able to send and receive frames without
the control word present.
In all cases the ingress PE MUST be aware of whether the egress
PE will send a control word over a specific PW. This may be
achieved by configuration of the PEs, or by signaling, as
defined in [PWE3-CTRL].
Additional Informational References:
[VCCV] T. D. Nadeau, R. Aggarwal, "Pseudo Wire Virtual Circuit
Connectivity Verification (VCCV)", draft-ietf-pwe3-vccv-07.txt,
August 2005. (work in progress)
[RFC2992] RFC-2992: Analysis of an Equal-Cost Multi-Path
Algorithm, C. Hopps, November 2000
[RFC4026] Andersson, L., Madsen, T. "Provider Provisioned
Virtual Private Network (VPN) Terminology", RFC
4026, March 2005.